The present invention relates to testing of thin film transistor (TFT) arrays, and more particularly to testing the functionality and reliability of such arrays.
Thin film transistor liquid crystal displays (TFT-LCD) for, e.g., television applications require brighter backlight for better image quality. FIG. 1 is a cross-sectional view of a TFT-LCD module assembly. The stack includes a polarizer layer 14 and optical film 12, followed by the TFT panel 10 above which liquid crystal layer 16 is formed, and then the backlight 20. Color filter 22 and polarizer 14 are disposed above liquid crystal layer 16. Brighter backlight increases the temperature of TFT-LCD during operation, thus resulting in an increase in the TFT-LCD off current Ioff. For good TFTs, the variation in Ioff as a function of temperature is relatively small, and does not affect the TFT-LCD image quality. However, In the case of defective TFTs, the off-current variation with temperature is large enough to deteriorate the TFT-LCD image quality during operation.
FIG. 2 is a cross-sectional view of a typical amorphous silicon (a-Si) TFT, which are typically N-channel enhancement type field effect transistors. Metal gate 40 is patterned first on a glass plate, followed by plasma enhanced CVD (chemical vapor deposition) deposition of a gate insulator dielectric material 42, such as silicon nitride (SiN), and layers of amorphous silicon semiconductor (a-Si) 44 and n+ a-Si 46. Source metal layer 48 and drain metal layer 50 are then patterned. Next, a passivation layer 52 is deposited over the whole structure. The n+ a-Si layer 46 acts as a low resistance ohmic contact for electrons to maximize the ON current. It also blocks injection of holes into the intrinsic layer to minimize the leakage current in the OFF state.
TFTs in flat panel displays operate as switches. If the gate voltage exceeds the threshold voltage, and a voltage is applied across the source and drain terminals, current flows from the source to drain. Gate layer 40 and a-Si layer 44 act as parallel plates of a capacitor between which dielectric SiN layer 42 is disposed.
Amorphous silicon is not very stable and its properties can be modified when exposed to strong illumination or injection of charge carriers. Over time, the interface between the a-Si layer 44 and SiN dielectric layer 42 can accumulate charge during normal operation of the TFT, thereby causing a shift over time of the threshold of the a-Si TFT. Under normal operating conditions, the threshold voltage shift during the ON-times is of the opposite polarity to that occurring during the OFF-times. Therefore, the shifts partially cancel one another. Furthermore, as long as the TFT drive can overcome this shift or variation, operation is not compromised.
FIG. 4A is an energy band diagram for an ideal amorphous semiconductor. For amorphous semiconductors, intrinsic localized states separated by the gap between the conduction band and valence band are established near the band edges. However, impurities, such as defects or dangling bonds within the amorphous material, populate the band gap with localized defect states, as shown in FIG. 4B. The localized defect states result in mobility of charges at nonzero temperatures due to thermally assisted tunneling between localized states. Thus, unlike normal semiconductors, the activation energy in amorphous semiconductors such as a-Si is related to the mobility gap rather than an energy gap.
The source-to-drain current ISD of a TFT is related to the density of states by the following expression:
      ln    ⁢                  ⁢          I      SD        ∝      [          A      -                                    E            C                    -                      E            F                    -                      q            ⁢                                                  ⁢                          Ψ              S                                      kT              ]  where A is a constant, EC is the conduction energy, EF is Fermi energy, ΨS is density of states, q is charge of electron, k is Boltzmann's constant, and T is temperature in Kelvin. FIG. 5 is an energy band diagram of the metal-insulator-semiconductor (MIS) structure, shown in FIG. 3.
With no voltages applied and at room temperature, the source-to-drain current ISD (IOFF) of the TFT has a small but nonzero value. As temperature increases, ISD rises, as illustrated in FIG. 6. In some TFT-LCD panel applications, such as televisions, in which the TFTs are illuminated and therefore heated by backlights, current Ioff normally remains sufficiently low.
During the processing of a TFT, a-Si is deposited through plasma enhanced chemical vapor deposition (PECVD) of silane or similar materials and methods. The resulting a-Si film is left with dangling bonds when the silicon-to-silicon bonds are broken. The dangling bonds are defects within the amorphous semiconductor layer and contribute to a nonzero density of states within the band gap, thereby resulting in the mobility of charges (off current). To minimize the density of states due to dangling bonds, the a-Si is hydrogenated. Typically for TFTs, a-Si:H film contains approximately 10 to 20% hydrogen.
During processing, however, the Si:H bond can be inadvertently broken. For example, during ion bombardment of the a-Si:H film, high energy ions can break the Si:H bond, leaving dangling bonds that lead to an increase in the density of states, and higher Ioff. Generation of high energy ions during processing can be due to poor or incorrect process parameters, and may result in a global plate (panel) effect rather than in a single, stand-alone TFT defect. In other words, a whole area of a panel rather than a single isolated TFT may have poor quality a-Si:H film.
A good TFT has a lower density of states in the band gap of a-Si:H and SiNx film, whereas a defective TFT has a higher density of states in the band gap of a-Si:H and SiNx film. As the temperature increases, the charges which are trapped in the band gap transport to the conduction band and contribute to TFT off current. Therefore, a defective TFT will have a larger Ioff at higher temperature (See FIG. 6).
Before the introduction of high illumination backlights for TFT-LCD televisions, the defects described above did not result in failed pixels, and the threshold voltage shifts due to turning the TFTs on and off canceled one another. Recently, the TFT-LCD panel manufacturers have noticed at module assembly that the powerful (and therefore heating) backlights cause such defects and adversely affect the yield. This type of defect cannot be repaired, but detecting it sufficiently early in the fabrication process is important to enable feedback and correction to the fabrication operational parameters to minimize loss.
One known method of detecting these defects takes advantage of the dependency of doff on temperature. Off current is measured while heat is applied to a TFT-LCD plate or panel that has been assembled into a module. In practice, however, such a method is difficult to implement at the high throughput rates required by the TFT-LCD manufacturers. Sampling is an acceptable technique, and currently manufacturers test fully assembled modules after the array is fabricated and after many of the assembly steps are completed. The main drawbacks associated with heating full panels and measuring Ioff are (a) the time required to heat the panels and (b) the complexity of the apparatus needed to accommodate the large-sized panels, which may be two meters long, and two meters wide.
A need continues to exist for a method and apparatus that detects this type of TFT defect during array testing of LCD plates and well before the process steps in which plates are divided into panels and assembled into modules.